1. Field of the Invention
The present invention relates to a thin film magnetic head having a dual spin-valve magneto-resistive element for reading as a signal a magnetic field strength of a magnetic recording medium or the like, and further relates to a head gimbal assembly and a hard disk drive each including such a thin film magnetic head.
2. Description of the Related Art
In recent years, following the improvement in areal recording density of a hard disk drive, improvement in performance of a thin film magnetic head has been required. As the thin film magnetic head, use has been widely made of a composite thin film magnetic head having a structure wherein a reproducing head comprising a read-only magneto-resistive effect element (hereinafter may also be referred to simply as an “MR element”), and a recording head comprising a write-only induction-type electromagnetic transducer element are stacked relative to a substrate.
As the MR element, there can be cited an AMR element using an anisotropic magneto-resistive effect, a GMR element using a giant magneto-resistive effect, a TMR element using a tunnel-type magneto-resistive effect, or the like.
As the GMR element, a spin-valve GMR element has often been used. The spin-valve GMR element comprises a nonmagnetic layer, a soft magnetic layer formed on one surface of the nonmagnetic layer, a ferromagnetic layer formed on the other surface of the nonmagnetic layer, and a pinning layer (generally an antiferromagnetic layer) formed on the ferromagnetic layer on its side apart from the nonmagnetic layer. The soft magnetic layer is a layer that acts to change its magnetization direction depending on a signal magnetic field from the exterior. The ferromagnetic layer is a layer of which a magnetization direction is fixed, i.e. which is subjected to exchange coupling by a magnetic field from the pinning layer (antiferromagnetic layer) so that the magnetization direction thereof is controlled in one direction (pinned direction).
Such an exchange coupling force and thermal stability of the exchange coupling force largely affect characteristics and reliability of the head and therefore it is required to produce as large an exchange coupling force as possible. It has been proposed that a ferromagnetic layer contacting with an antiferromagnetic layer is formed into a three-layer structure of ferromagnetic layer/nonmagnetic metal layer/ferromagnetic layer, different from a conventional single-layer structure, so as to induce strong exchange coupling between the two ferromagnetic layers (synthetic ferrimagnetic structure), thereby effectively increasing the exchange coupling from the antiferromagnetic layer (JP-A-2000-137906, JP-A-2000-113418).
Further, in terms of improving the output, a proposal has been made for a dual spin-valve structure wherein two conventional spin-valve sensors are stacked together (JP-A-H09-147326, JP-A-2002-185060). Specifically, since two nonmagnetic layers of Cu or the like exist in the dual spin-valve structure, the number of interfaces for improving the GMR change rate increases to four and more sense current can flow in total. Further, an element height (MR height) corresponding to an inward depth from an ABS (Air Bearing Surface) can be reduced as compared with that of a single spin-valve structure.
However, in the dual spin-valve structure, antiferromagnetic layers for fixing a magnetization direction need to be formed on a lower-layer side and an upper-layer side, respectively, i.e. two antiferromagnetic layers in total need to be formed. As a result of assiduous studies by the present inventors about film structures of these antiferromagnetic layers, it has been found as one of hints to reach the present invention that the antiferromagnetic layer near an underlayer can be formed as an excellent film with high reliability under the influence of the underlayer, while the other antiferromagnetic layer located on the upper-layer side tends to be a film with low reliability of which the heat resistance and impact resistance are insufficient as different from the film quality of the antiferromagnetic layer located below.
Consequently, there is a tendency that the pinning effect achieved by the antiferromagnetic layer located on the upper-layer side includes many unstable factors, thus resulting in insufficient reliability in pinning direction.
In addition to such a reason, in order to further reduce the thickness of a dual spin-valve structure film, a proposal has been made for a technique of producing a pinning effect for a ferromagnetic layer on one side without employing a corresponding antiferromagnetic layer, i.e. by the use of another method (JP-A-2000-276714, JP-A-2000-149229). More specifically, there has been proposed a spin-valve magnetic sensor that exhibits a pinning bias characteristic according to magnetostatic coupling at end portions of magnetic films and a current bias magnetic field generated by a sense current, i.e. without a pinning function achieved by the antiferromagnetic film or the like. However, in the proposed technique, the pinning operation of fixing a magnetization direction of the ferromagnetic layer using only the magnetic field generated by the sense current lacks stability in pinning fixation in terms of the current state of the art, and therefore, it can be said that commercialization is not practical. It is also suggested to dispose an antiferromagnetic layer on only one side of a dual spin-valve structure film and utilize a static magnetic field leaking from a ferromagnetic layer exchange-coupled to the antiferromagnetic layer.
However, the present situation is that there is no indication of a specific structure for realizing a product of which the reproduction output is large while further gap narrowing and track narrowing are achieved and of which the reliability is high with enhanced stability in pinning fixation.
The present invention has been conceived under these circumstances and has an object to provide a thin film magnetic head of which the reproduction output is large while further gap narrowing and track narrowing are achieved and of which the reliability is high with enhanced stability in pinning fixation, and has a further object to provide a head gimbal assembly and a hard disk drive each comprising such an improved thin film magnetic head.
Particularly, with respect to the stability in pinning fixation, it is an object of the present invention to provide a thin film magnetic head having properties such that even when (1) there is HDI (Hard Disk Interface) damage due to contact between the thin film magnetic head and a recording medium, or the like during operation of the thin film magnetic head, (2) there is processing stress caused by cutting a wafer into bars each including many thin film magnetic heads and further cutting each bar into the individual magnetic heads, or the like, a pinned direction of a pinned magnetic layer can be maintained in a proper direction without being reversed so that the stability and reliability of the operation are ensured, i.e. the rate of pinning reversal failure, where the polarity is reversed during the operation, is small and the rate of pinned direction failure, where the pinned direction does not coincide with a required direction after polishing, is small.